This invention relates to a spring clutch for transmitting the rotation of an input member to an output member, and for shutting off the transmission of the rotation of the output member to the input member.
Generally, in an automotive engine, the crankshaft is subjected to change in its angular velocity in one rotation since driving energy is produced during the explosion stroke. When an engine accessory having a large inertia such as an alternator is driven by such a crankshaft through a belt transmission, slip occurs between a pulley mounted on the rotary shaft and the belt when the angular velocity of the crankshaft decreases and if the rotating speed of the rotary shaft of the engine accessory exceeds the angular velocity of the crankshaft.
Also, when the engine is changed over from high-speed to low-speed and the velocity of the belt drops, slip occurs between the pulley and the belt since the engine accessory such as an alternator maintains high-speed rotation due to inertia. During such slip, abnormal noise is produced, or the belt wears and its durability thus lowers.
In order to solve such a problem, in accordance with a clutch pulley device as disclosed in JP patent publication 9-119509, a spring clutch is provided between a pulley hub and a pulley. The pulley hub is mounted on the rotary shaft of an alternator, and the pulley, which rotates by contact with a belt, is mounted so as to be rotatable relative to the pulley hub to transmit the rotation of the pulley from the pulley hub to the rotary shaft through the spring clutch. When the rotating speed of the rotary shaft exceeds that of the pulley, transmission of the rotation from the rotary shaft to the pulley is shut off by the spring clutch.
The spring clutch is formed with cylindrical surfaces having the same inner diameter as the inner peripheries of the pulley hub and the pulley, and a coil spring having a larger outer diameter than the diameter of the cylindrical surfaces is mounted so as to straddle the cylindrical surfaces of the pulley hub and pulley. With such a formation of the spring clutch, the diameter of the coil spring increases as the pulley rotates, which thereby increases the frictional force of the coil spring on the cylindrical surfaces so as to transmit the rotation of the pulley to the pulley hub through the coil spring.
Also, when the rotating speed of the pulley hub exceeds that of the pulley, the coil spring is wound to weaken its frictional force against the cylindrical surfaces, thereby causing slip between the cylindrical surfaces and the coil spring.
If an existing coil spring is used as the coil spring instead of a coil spring as described above in the preceeding two paragraphs, burrs are formed on the end faces in many cases since the end faces of existing coil springs are formed by shearing.
If a coil spring having burrs formed on its end faces is mounted between the cylindrical surfaces of the pulley hub and the pulley, the burrs will engage the cylindrical surfaces, thus making the engagement between the outer periphery of the coil spring and the cylindrical surfaces unstable. Also, since a coil spring having burrs on its end faces cannot slide smoothly on the cylindrical surfaces during the shut-off of the transmission of the reverse input torque resulting from a shrinkage of the coil spring, the spring clutch cannot stably perform engaging and idling functions.
Also, since the surface pressure increases at the contact portions between the cylindrical surfaces and the burrs, the cylindrical surfaces will be abraded abnormally.
An object of this invention is to provide a spring clutch which can perform stable engaging and idling functions and which can suppress abnormal wear of torque-transmitting surfaces against which the coil spring is pressed.
According to this invention, there is provided a spring clutch comprising an input member, an output member mounted coaxially with the input member in which the input member and the output member are each formed with a cylindrical torque-transmitting surface having substantially the same diameter, and a coil spring mounted so as to be pressed against and to straddle, i.e. engage with, the torque-transmitting surfaces, wherein the coil spring has two end faces each extending to an edge, and each of the edges is chamfered at least extending along an outer periphery of a portion of the edges which oppose the torque-transmitting surfaces.
The chamfer may be of a shape formed by grinding an edge in a straight line or arcuately, or by rounding an edge, e.g. by tumbling, but not by cutting.
By providing a chamfer on an end face formed by cutting the coil spring, it is possible to prevent the edge of the end face of the coil spring from being brought into strong frictional engagement with the cylindrical surfaces of the input and output members.
Thus, in a spring clutch in which cylindrical torque-transmitting surfaces are formed on the inner peripheries of the input member and the output member and a coil spring is mounted so as to straddle and thereby engage with these torque-transmitting surfaces, the entire periphery of the coil spring is brought into frictional engagement with the cylindrical surfaces when the diameter of the coil spring increases. A stable engaging function is thereby obtained, and during idling, due to a reduction in the diameter of the coil spring, abnormal wear of the torque-transmitting surfaces is avoided.
The spring clutch is not limited to one in which cylindrical torque-transmitting surfaces are formed on the inner peripheries of the input member and the output member.
For example, the spring clutch may be one in which cylindrical torque-transmitting surfaces of substantially the same diameter are formed on the outer peripheries of the input member and the output member, and a coil spring is pressed against these torque-transmitting surfaces. In such a spring clutch, rotation of the input member is transmitted to the output member by reducing the diameter of the coil spring, and when the rotating speed of the output member exceeds that of the input member, the diameter of the coil spring is increased to prevent the rotation of the output member from being transmitted to the input member.
The coil spring may be made from wire material having a circular cross-section or a square cross-section When a coil spring made from a wire material having a square cross-section is used, since the entire outer or inner peripheral surface of the coil spring contacts the torque-transmitting surfaces by frictional engagement of the coil spring during torque transmission, it is possible to obtain a spring clutch which has a large load capacity and which lowers the wear of the torque-transmitting, surfaces, because the surface pressure is low compared to a coil spring made from a wire material of a circular cross-section.
With a coil spring made from a square cross-section wire material, its diameter increases or decreases with the adjacent coil portions of the coil spring in close contact with each other, and while the diameter of the wire material is increasing or decreasing, the coil spring is deformed while twisting. Smooth twisting deformation, however, is not possible because four corners of the section of the wire material are deformed while abrading sides of the adjacent coil portions.
Thus, in the case of a coil spring made from a square wire material, chamfers are preferably provided on the four corners of the end face section of the wire material.